MRI is an effective, non-invasive magnetic imaging technique for generating sharp images of the internal anatomy of the human body, which provides an efficient means for diagnosing disorders such as neurological and cardiac abnormalities and for spotting tumors and the like. Briefly, the patient is placed within the center of a large superconducting magnetic that generates a powerful static magnetic field. The static magnetic field causes protons within tissues of the body to align with an axis of the static field. A pulsed radio-frequency (RF) magnetic field is then applied causing the protons to begin to precess around the axis of the static field. Pulsed gradient magnetic fields are then applied to cause the protons within selected locations of the body to emit RF signals, which are detected by sensors of the MRI system. Based on the RF signals emitted by the protons, the MRI system then generates a precise image of the selected locations of the body, typically image slices of organs of interest.
However, MRI procedures are problematic for patients with implantable medical devices such as pacemakers and ICDs. One of the significant problems or risks is that the strong RF fields of the MRI can induce currents through the lead system of the implantable device into the surrounding tissue, resulting in Joule heating in the cardiac tissue around the electrodes of leads and potentially damaging adjacent tissue. Indeed, the temperature at the tip of an implanted lead has been found to increase as much as 60° Celsius (C.) during an MRI test with the lead immersed in a gel phantom in a non-clinical configuration. Although such a dramatic increase is probably unlikely within a clinical system wherein leads are properly implanted, even a temperature increase of only about 8°-13° C. might cause myocardial tissue damage.
Furthermore, any significant heating of cardiac tissue near lead electrodes can affect the pacing and sensing parameters associated with the tissue near the electrode, thus potentially preventing pacing pulses from being properly captured within the heart of the patient and/or preventing intrinsic electrical events from being properly sensed by the device. The latter might result, depending upon the circumstances, in therapy being improperly delivered or improperly withheld. Another significant concern is that any currents induced in the lead system can potentially generate voltages within cardiac tissue comparable in amplitude and duration to stimulation pulses and hence might trigger unwanted contractions of heart tissue. The rate of such contractions can be extremely high, posing significant clinical risks to patients. Therefore, there is a need to reduce heating in the leads of implantable medical devices, especially pacemakers and ICDs, and to also reduce the risks of improper tissue stimulation during an MRI, which is referred to herein as MRI-induced pacing.
Various techniques have been developed to address these or other related concerns. See, for example, the following patents and patent applications: U.S. patent application Ser. No. 11/943,499, filed Nov. 20, 2007, of Zhao et al., entitled “RF Filter Packaging for Coaxial Implantable Medical Device Lead to Reduce Lead Heating during MRI” (abandoned); U.S. Published Patent Application 2009/0281592, filed May 8, 2008, of Vase, entitled “Shaft-mounted RF Filtering Elements for Implantable Medical Device Lead to Reduce Lead Heating During MRI”; U.S. patent application Ser. No. 11/860,342, filed Sep. 27, 2007, of Min et al., entitled “Systems and Methods for using Capacitive Elements to Reduce Heating within Implantable Medical Device Leads during an MRI”; U.S. patent application Ser. No. 12/042,605, filed Mar. 5, 2009, of Mouchawar et al., entitled “Systems and Methods for using Resistive Elements and Switching Systems to Reduce Heating within Implantable Medical Device Leads during an MRI”; and U.S. patent application Ser. No. 11/963,243, filed Dec. 21, 2007, of Vase et al., entitled “MEMS-based RF Filtering Devices for Implantable Medical Device Leads to Reduce Lead Heating during MRI.”
See, also, U.S. patent application Ser. No. 12/257,263, filed Oct. 23, 2008, of Min, entitled “Systems and Methods for Exploiting the Ring Conductor of a Coaxial Implantable Medical Device Lead to provide RF Shielding during an MRI to Reduce Lead Heating”; U.S. Published Patent Application 2010/0106227, filed Oct. 23, 2008, of Min, entitled “Systems and Methods for Disconnecting Electrodes of Leads of Implantable Medical Devices during an MRI to Reduce Lead Heating while also providing RF Shielding”; U.S. Published Patent Application 2010/0121179, of Min et al., filed Nov. 13, 2008, entitled “Systems And Methods For Reducing RF Power or Adjusting Flip Angles During an MRI For Patients with Implantable Medical Devices”; and U.S. Published Patent Application 2010/0106214 of Min et al., filed Oct. 23, 2008, entitled “Systems and Methods for Exploiting the Tip or Ring Conductor of an Implantable Medical Device Lead during an MRI to Reduce Lead Heating and the Risks of MRI-Induced Stimulation”.
At least some of these techniques are directed to installing RF filters, such as inductive (L) filters or inductive-capacitive (LC) filters, within the leads for use in filtering signals at frequencies associated with the RF fields of MRIs. It is particularly desirable to select or control the inductance (L), parasitic capacitance (Cs) and parasitic resistance (Rs) of such devices to attain a high target impedance (e.g. at least 1000 ohms) at RF to achieve effective heat reduction. See, for example, U.S. patent application Ser. No. 11/955,268, filed Dec. 12, 2007, of Min, entitled “Systems and Methods for Determining Inductance and Capacitance Values for use with LC Filters within Implantable Medical Device Leads to Reduce Lead Heating during an MRI”; U.S. Published Patent Application 2010/0138192, of Min et al., filed Dec. 1, 2008, entitled “Systems and Methods for Selecting Components for Use in RF Filters within Implantable Medical Device Leads based on Inductance, Parasitic Capacitance and Parasitic Resistance.”
Although these techniques are helpful in reducing lead heating due to MRI fields, there is room for further improvement. In particular would be desirable to provide for LC filtering with smaller package size and/or better electrical and mechanical reliability. It is to these ends that aspects of the invention are generally directed.